1. Field of the Invention
The present invention relates to a plane display unit in which pixels using light-emitting elements are arranged in a plural number on a substrate, and to a plane display apparatus using the plane display unit.
2. Description of the Related Art
A plane display unit having a plurality of pixels of LEDs (light-emitting diodes) arranged on a substrate has been widely used by itself as a display unit for constituting a screen or as a display unit for constituting a large screen by being arranged in a plural number like tiles.
Among the conventional LED plane display units, the one which displays the image upon inputting video signals is so constructed, as shown in FIG. 7, that a pixel is formed by a total of three light-emitting portions (in the drawing, these light-emitting portions are denoted by R, G and B), i.e., a light-emitting portion having a red LED pellet (LED chip), a light-emitting portion having a green LED pellet and a light-emitting portion having a blue LED pellet, and a plurality of the pixels are arranged in the transverse direction and in the longitudinal direction.
Further, among the LED plane display units, the one which displays various guide data in the form of characters is so constructed, as shown in FIG. 8, that LED pellets 21 and 22 of two different colors (e.g., yellowish green color and red color) are mounted on a substrate 23 being brought close to each other, the circumference of these LED pellets 21 and 22 is surrounded by a reflector frame 24, the upper portion thereof is covered with a disk-like transparent scattering plate 25 to form a pixel 26 having a circular light-emitting surface as shown in FIG. 9, and a plurality of these pixels 26 are arranged in the transverse direction and in the longitudinal direction as shown.
In the LED plane display unit for displaying image shown in FIG. 7, however, each light-emitting portion emits light of only one color. Unless a person views the screen from a certain amount of distance, therefore, the colors of the individual light-emitting portions appear separately, which is a deterioration in the picture quality.
In the future, however, it is considered that the LED plane display unit for displaying image might be installed not only on a place at where it is viewed from a distance, such as on the wall of a building outdoors but also on a place at where it is viewed from a close range and, hence, that the above inconvenience may become conspicuous.
This inconvenience can be solved by, for example, narrowing the gap among the light-emitting portions in a pixel as shown in FIG. 10. When the gap among the light-emitting portions is narrowed in each pixel under a fixed pixel pitch condition, however, the gap increases among the pixels as shown. When the screen is viewed from a close range, therefore, the individual pixels can be clearly discerned and the picture may appear as a set of so-called lumpy dots, which is a deterioration in the picture quality, too.
With the element of the LED plane display unit for displaying characters as shown in FIGS. 8 and 9, on the other hand, light from the LED pellet 21 and light from the LED pellet 22 are scattered through the scattering plate 25. Therefore, the light-emitting surface emits light of a color which is a mixture of yellowish green color and red color. Even when the screen is viewed from a close range, therefore, yellowish green color and red color do not appear separately. However, since the light-emitting surface is of a circular shape, the areas among the light-emitting surfaces are never small compared to the areas of the light-emitting surfaces as shown in FIG. 9. When the screen is viewed from a close range, therefore, the individual pixels 26 are clearly discerned and the character appears as a set of lumpy dots, which, still, is a deterioration in the picture quality.
The LED plane display elements for displaying characters have now been installed in many places at where it can be seen from a close range, such as in the trains, letting the above-mentioned inconvenience to appear conspicuously.
It is an object of the present invention to provide a plane display unit which improves the separate and distinguished appearance of the pixels that use light-emitting elements such as LEDs and improves the appearance of a set of visible dots even when the screen displaying pictures and characters is viewed from a close range, as well as to provide a plane display apparatus using the above plane display unit.
The plane display unit of this invention and the plane display apparatus in which the above units are arranged in plural numbers like a matrix of this invention, are characterized in that the pixels having light-emitting surfaces of nearly a rhombic shape are arranged on a substrate in a direction inclined toward the right and in a direction inclined toward the left with the sides of the light-emitting surfaces being opposed to each other.
The rhombic shape, in general, stands for a quadrilateral of which the sides all have an equal length, and the angle subtended by the neighboring sides is not 90xc2x0. In this specification, however, xe2x80x9cnearly rhombic shapexe2x80x9d encompasses not only the rhombic shape but also those close to the rhombic shape, such as a quadrilateral (i.e., a square that is inclined by 45xc2x0) of which the sides all have an equal length and the angle subtended by the neighboring sides is 90xc2x0.
In this plane display unit, plural pixels having a light-emitting surface of nearly a rhombic shape are arranged on the substrate in a direction inclined toward the right and in a direction inclined toward the left with the sides of the light-emitting surfaces being opposed to each other or, in other words, are so arranged that the gaps among the light-emitting surfaces form an inclined lattice shape.
Due to the shape and arrangement of the light-emitting surfaces, the plane display unit has a small ratio of the areas among the light-emitting surfaces to the areas of the light-emitting surfaces. Even when viewed from a close range, therefor, the individual pixels do not appear distinctly as compared to the conventional plane display unit shown in FIG. 9. When viewed in a close range, therefore, the image and characters do not appear as a set of visible pixels.
In the plane display unit, further, the pixel pitch in the transverse direction of the screen is not the pixel pitch among the pixels neighboring in the transverse direction but is the transverse direction component of the pixel pitch among the pixels neighboring in a inclined direction. Therefore, the pixel pitch becomes shorter than the pixel pitch in the transverse direction of the plane display unit in which the pixels having circular or square light-emitting surfaces are arranged in the transverse direction and in the longitudinal direction. Similarly, the pixel pitch in the longitudinal direction on the screen is not apparently the pixel pitch among the pixels neighboring in the longitudinal direction but is the longitudinal direction component of the pixel pitch among the pixels neighboring in the inclined direction. Therefore, the pixel pitch becomes shorter than the pixel pitch in the longitudinal direction of the plane display unit on which the pixels having circular or square light-emitting surfaces are arranged in the transverse direction and in the longitudinal direction.
As described above, the pixel pitch which is apparently small in the transverse direction and in the longitudinal direction on the screen, contributes to enhancing the transverse resolution and the longitudinal resolution.
In the plane display unit, it is desired that the surface of the substrate has a shape nearly the same as the contours of the light-emitting surfaces of all pixels on the substrate or, in other words, has a saw-tooth shape to meet the contours of the light-emitting surfaces instead of the square shape and that the distance from the light-emitting surface located closest to the edge of the substrate surface to the edge of the substrate surface, is not larger than about one-half the distance between the light-emitting surfaces having sides opposed to each other on the substrate.
In arranging the plane display units in a plural number like a tile to constitute a large screen, therefore, the gap between the light-emitting surfaces at the boundary of the neighboring plane display units can be set to be equal to the gap among the light-emitting surfaces on the substrate of the plane display unit, making it possible to maintain uniformity in the pixel pitch even at the boundaries.
As another example, further, the surface of the substrate in the plane display unit may have a size smaller than the size of contours of the light-emitting surfaces of all pixels on the substrate. In this case, too, the gap among the light-emitting surfaces at the boundaries of the neighboring plane display units can be set to be nearly equal to the gap among the light-emitting surfaces on the substrate of the plane display unit at the time of constituting a large screen by arranging plural plane display units like a tile, making it possible to maintain a uniform pixel pitch even at the boundaries.
In the plane display unit, it is desired that the number of the pixels on the substrate in the transverse direction is equal to the number of the pixels on the substrate in the longitudinal direction.
When the plane display units are arranged in the transverse direction and in the longitudinal direction in numbers of 4n and 3n (n is an integer), then, there is constituted a screen having an aspect ratio of 4:3. When the plane display units are arranged in the transverse direction and in the longitudinal direction in numbers of 16n and 9n, then, there is constituted a screen having an aspect ratio of 16:9. Thus, the screen having a desired aspect ratio can be easily constituted.
In the plane display unit, when LEDs are used as light-emitting elements, it is desired to form the pixels by covering the LEDs mounted on the substrate with a transparent material that contains a material that scatters light. Upon covering the LEDs with the transparent material containing the scattering material, light from the LEDs are homogeneously scattered over the whole light-emitting surfaces, and the brightness becomes uniform on the light-emitting surfaces.
In the plane display unit, further, it is desired to use a material having a refractive index different from that of the transparent material, as the scattering material. Upon refracting light from the LED by using the scattering material as described above, a decreased amount of light travels in the directions other than the light-emitting surfaces among the light arriving at the scattering material from the LEDs and, hence, the light-emitting efficiency is improved. Among the light incident on the scattering material from the external side, further, less light returns back to the external side, preventing a drop in the contrast caused by light incident from the external side.
In the plane display unit, it is desired to use the material having a refractive index smaller than that of the transparent material, as the scattering material. In selecting the material used as the scattering material and the material used as the transparent material, the material having a relatively large refractive index can be used as the transparent material. This makes it possible to decrease the difference in the refractive index between the LED (usually, having a refractive index of as high as 4 to 5) and the transparent material and, hence, to decrease the ratio of total reflection of light from the LEDs reflected by the boundary surface between the LEDs and the transparent material and, accordingly, to suppress a drop in the light emitting efficiency caused by the total reflection.
In the plane display unit, further, it is desired to surround the light-emitting diodes with a material that reflects light and to bury space surrounded by the reflecting material in the transparent material. Then, even light that has traveled in the directions other than the light-emitting surfaces among light that has arrived at the scattering material from the LEDs, is caused to travel again in the direction of the light-emitting surfaces by being reflected by the reflecting material, contributing to further enhancing the light-emitting efficiency.
In the plane display unit, further, it is desired to form the pixels by covering plural light-emitting diodes of different colors with the above-mentioned transparent material. Upon covering the plurality of LEDs of different colors with the transparent material, light from the individual LEDs is scattered through the scattering material, and the light-emitting surfaces emit light of a color of a mixture of the above colors. Even when the screen is viewed in a close range, therefore, the colors do not appear separately.